We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10 K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T ¼ 1.5 K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag. DOI: 10.1103/PhysRevLett.117.046803 Interaction between isolated electron systems in close proximity can produce a wealth of novel phenomena. A particularly striking example is frictional drag, where charge current (I Drive ) flowing in one (drive) layer induces a voltage drop in the opposite (drag) layer, V Drag ¼ R D I Drive . At the heart of the transresistance R D are interlayer couplings without particle exchange which can be mediated by, e.g., momentum exchange Extensive experimental effort has been devoted to probing drag in electron-hole double layers, using GaAs electronhole double layers [7,8], graphene double layers [9,10], and, most recently, graphene-GaAs double layers [11], motivated in part by the search for equilibrium indirect exciton condensates. A common thread in these experiments is an anomalous R D that increases with reducing T, along with a breakdown of layer reciprocity when interchanging the drive and drag layers [7,8,11]. In this regard, double bilayer graphene separated by a thin hexagonal boron nitride (hBN) is a particularly compelling system. The near parabolic energy-momentum dispersion in bilayer graphene allows the Coulomb to kinetic energy ratio to be tuned via density, unlike monolayer graphene, where this ratio is fixed [12]. Moreover, the availability of ultrathin dielectrics allows double layers to be realized with interlayer spacing (d) down to a few nanometers, granting access to the strong coupling regime d ≪ l, where l is the interparticle distance. This effectively nests the two isolated electronic systems in the same plane. Here, we investigate the frictional drag in double bilayer graphene heterostructures, consisting of two bilayer graphene separated by a 2-5 nm thick interlayer hBN dielectric, which allows us to explore the drag in a wide range of layer densities and for all combinations of carrier polarity. Strikingly, we find a giant and negative drag resistivity at charge neutrality, comparable to the layer resistivity at the lowest T.The samples [ Fig. 1(a)] are fabricated using a layer-bylayer transfer process similar to samples discussed in Ref. [13]. The layer densities are tuned using a combination of back-gate (V BG ), and interlayer bias applied on the top bilayer (V TL ) [14]. The top (ρ T ) and bottom (ρ B ) bilayer resistivities, as well as the frictional drag, are probed using small signal, low frequency lock-in techniques. We...

Phonon-polaritons (PhPs) arise from the strong coupling of photons to optical phonons. They offer light confinement and harnessing below the diffraction limit for applications including sensing, imaging, superlensing, and photonics-based communications. However, structures consisting of both suspended and supported hyperbolic materials on periodic dielectric substrates are yet to be explored. Here we investigate phonon-polaritonic crystals (PPCs) that incorporate hyperbolic hexagonal boron nitride (hBN) to a silicon-based photonic crystal. By using the near-field excitation in scattering-type scanning near-field optical microscopy (s-SNOM), we resolved two types of repetitive local field distribution patterns resembling the Archimedean-like tiling on hBN-based PPCs, i.e., dipolar-like field distributions and highly dispersive PhP interference patterns. We demonstrate the tunability of PPC band structures by varying the thickness of hyperbolic materials, supported by numerical simulations. Lastly, we conducted scattering-type nanoIR spectroscopy to confirm the interaction of hBN with photonic crystals. The introduced PPCs will provide the base for fabricating essential subdiffraction components of advanced optical systems in the mid-IR range.

Monolayer WTe 2 is predicted to be a quantum spin Hall insulator (QSHI), and its quantized edge transport has recently been demonstrated. However, one of the essential properties of a QSHI, spin-momentum locking of the helical edge states, has yet to be experimentally validated. Here, we measure and observe gate-controlled anisotropic magnetoresistance (AMR) in monolayer WTe 2 devices. Electrically tuning the Fermi energy into the band gap, a large in-plane AMR is observed and the minimum of the in-plane AMR occurs when the applied magnetic field is perpendicular to the current direction. In line with the experimental observations, the theoretical predictions based on the band structure of monolayer WTe 2 demonstrate that the AMR effect originates from spin-momentum locking in the helical edge states of monolayer WTe 2 . Our findings reveal that the spin quantization axis of the helical edge states in monolayer WTe 2 can be precisely determined from AMR measurements.